Moulding transfer

In a transfer mould, there is a special plasticising area (cylinder) as well as cavities. 

Practically, transfer moulding is used for the processing of thermosets only, as there are no advantages with the proeessing of thermoplasties eompared to the injection moulding method. 

Transfer moulding is a process in which molten resin is transferred from a melt pot into a mould that is hotter than the resin. This is rnilike injection moulding where the mould is cooler. The higher temperature is important for filling long moulds to maintain a consistent wall thickness and to compensate for shrinkage. Once the plastic has cooled, it is removed and the next charge is loaded. 

The success of transfer moulding prompted further developments in this area and clearly it was only a relatively small step to an injection moulding process for thermosets as described in Section 4.3.10. 

A major disadvantage is that the mould temperature must be kept relatively low so that reaction times are lengthy if the polymer surface in contact with the hot mould cures too quickly (before the rest of the charge is heated and therefore liquefied) a poor moulding can result and the mould cavity pressure may then be inhomogeneous. These problems are reduced by preheated screw feed and also in the related technique of transfer moulding. 

At very low strain rates polymer melts flow approximately as incompressible Newtonian fluids. Show that under these conditions the elongational viscosity A and shear viscosity are related by 

A film of polymer melt is stretched at a rate of elongation e, while constrained so that its width remains constant. Show that, under the conditions referred to in Problem 

A thread of molten polymer is extruded at a volume flow rate Q and under a tension F. Assuming isothermal conditions and Newtonian flow with elongational viscosity A, show that the velocity t of the thread incre s with distance z from the die as follows 

A tubular film of linear polyethylene was extruded with straight sides (achieved by applying internal air pressure). The volume flow rate was 7 x 10 m /s, and the haul- 

The last term in this equation is simpfy the velocity, whicdi for a volume flow-rate Q must be AQ/ttD, and the gradient of Z is clearfy 2 tan a, where a is the half-angle of the conical die chaimel. The rate of radial strain becomes 

47 An element of melt passes through a cylindrical die. The die channel narrows, causing the element to be squeezed and to elongate. If the rate of elongation is too high, melt fracture will occur. 

The melt is incompressible to a good approximation hence the axial and radial strain-rates are related  

48 Graph showing the melt viscosity of polystyrene at 217°C, plotted versus weight-average RMM. Two regimes are apparent. Below the critical RMM the gradient of the log-log plot is 1 above Mg it is 3.4. For polystyrene, Mg 39000. 

Quality-control tests and important process variables 

Here the transfer-moulding process where by the epoxy-silica charge is transferred to the mould to encapsulate the integrated circuit is shown. Note that in this case the fragile integrated-circuit pattern is placed inside the heated mould (typically at about 170-180 °C) 

Manzione et al. (1988) presented an empirical model for transfer moulding of filled epoxyresin systems for integrated-circuit encapsulation. The following chemorheological model (combining temperature, shear and cure effects) was used to aid flow-balancing calculations  

Nguyen et al. (1992, 1993) highlighted a full process model for transfer moulding of highly filled epoxies in integrated-circuit encapsulation. This model used the following kinetic and chemorheological models  

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Encapsulation of semiconductors. The usual material is epoxide resin (see Chapter 26) and the preferred method transfer moulding. It has been estimated that by 1980 annual production of such encapsulated parts exceeded 10 billion units. 

Urea-formaldehyde moulding powders may be moulded without difficulty on conventional compression and transfer moulding equipment. The powders, however, have limited storage life. They should thus be stored in a cool place and, where possible, used within a few months of manufacture. 

Urea-formaldehyde moulding powders may be transfer moulded. Pressures of 4-10 ton/in (60-150 MPa), calculated on the area of the transfer pot, are generally recommended. 

Melamine-based compositions are easily moulded in conventional compression and transfer-moulding equipment. Moulding temperatures are usually in the range 145-165°C and moulding pressures 2-4 ton/in (30-60 MPa). In transfer moulding pressures of 5-lOton/in (75-150MPa) are used. An in thick moulding required about 2 minutes cure at 150°C but shorter times are possible with preheated powder. 

For some years there has been concern at the amount of styrene vapour in workshops preparing reinforced polyester laminates. More recently this has increased interest in polyester-polyurethane hybrids and in the further development of closed moulding and resin transfer moulding techniques as well as greater use of lower styrene levels. 

The method of manufacture of solid rubber products by simultaneous shaping and vulcanising. An uncured blank of rubber compound is placed in the bottom half of a mould, the top half of the mould placed in position and the mould placed in a heated hydraulic press. The press performs the dual function of both closing and heating the mould. See Transfer Moulding and Injection Moulding. 

The process of making rubber products by shaping in a mould vulcanisation is generally effected at the same time. See Compression Moulding, Injection Moulding and Transfer Moulding. 

In transfer moulding, the channels in the mould which transfer mbber compound to the actual moulding cavities. 

An injection machine and mould cost considerably more than a compression press and compression mould. The main economical advantages of injection moulding lie with long production runs and complicated mouldings. For short runs compression or transfer moulding may still be the most suitable process. 

Microwave units are available for both continuous vulcanisation systems, for preheating of moulding blanks for compression and transfer moulding, and for heating large bales of frozen rubber prior to mixing. 

Injection and transfer moulds do not require any provision for excess material flowing out of the mould, simply an escape route for the volume of air in the cavities, which the parting faces provide. 

Transfer moulding uses an additional pot/ram section built into the top of the mould. A pad of rubber compound sufficient to fill the cavities, plus an allowance for the transfer pad remaining in the transfer pot, is put into the pot and the mass transferred into the individual cavities. 

The other method is to use a transfer mould where the blank is placed in a pot portion of the mould and then picks up further heat as it passes into the cavity. This method is frequently adopted for rubber-to-metal bonded, large and complex components. 

In commercial use are a wide variety of machines which transfer fixed volumes of compound to clamped moulds, by the operation of a ram in a cylinder fed with pre-heated slugs. These machines are capable of high speed semi-automatic cycling and are a great advance over the use of loose transfer moulds in conventional presses. The components which are produced are to a large degree flash-free, it only being necessary, in the majority of cases, to remove the injection feed and runner system. 

Polystyrene insulation on magnet wire 0.29 Encapsulated with B-staged aromatic amine cured aliphatic amine cured bisphenol A-epichlorohydrin epoxide (epoxy transfer moulded). Impregnate. 

Compression-transfer moulding (not much used for thermoplastics) o is suited for medium output... 

Transfer moulding involves the distribution of the uncured stock from one part of the mould called as pot into the actual mould cavity. This process permits the moulding of intricate shapes or the introduction of inserts like metals in many composite products. These procedures are difficult in compression moulds. Although these moulds are relatively more expensive than compression moulds, the actual process permits shorter cure times through the use of higher temperatures and better heat transfer which is obtained due to higher pressure applied to force the compound into the mould. 

Three types of moulding processes are available - compression, transfer and injection moulding. In compression moulding the rubber blank is placed directly into the cavity of the mould where it is heated by conduction which causes rubber flow by application of pressure. Transfer moulding uses prewarmed rubber which is heated during transfer and forced through small orifices into the mould cavities in a three-part mould. In the injection moulding process, the rubber compound is pushed under pressure from an injection head where it has been heated and plasticized into a closed heated mould where cure is completed. 

No Compression moulding Transfer moulding Injection moulding... 

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